pgsdeepak:
I currently own a EOS 40D, 17-40mm F4L, 100MM Macro Non L, 70-300mm IS Non L and 28-135mm Kit lens. I have made a decision to upgrade my 40D to 5D Mark III kit. Since the rebate is ending today, I decided to wait until the next rebate or price drop if that happens. I was also thinking of adding the 70-200mm F4 IS L lens to my collection. Then another thought came to my mind. Sell the 70-300mm IS USM and buy the 70-300mm IS L or 100-400MM IS L and forget about the 70-200 mm F4 IS L. What would be the best decision, 70-300MM L, 100-400MML or keep my 70-300 non L and buy 70-200mm IS f/4 L. I considered 70-200mm L for its sharpness even though I decided to keep the 70-300MM for the reach. I never used the 70-300mm L or 100-400mm L. I read the reviews on thedigitalpicture.com and they are high on 70-200mm lenses. Please advice. thanks in advance.Also any idea, how much I can ask for the 70-300mm IS USM? its 4.5yrs old and is in excellent condition. this is the first time I am trying to sell a lens. thought of starting with Craigslist. any other suggestion?

neuroanatomist:
The 70-200/4L IS, 70-300L, and 100-400L are all excellent. Your choice should be determined by whether you need 200mm, 300mm, or 400mm.

The 70-300 non-L seems to be listed on my local CL frequently, in the $300-350 range (higher sometimes, but I highly doubt those sell).

dr croubie:
Keeping the 70-300 non-L and buying one of the 70-200L variants is not a good idea, imho. The 70-300 non-L is good in the 70-200 range, but goes rapidly downhill towards 300mm. So if you keep that and buy a 70-200L, then you've got a great lens at 70-200mm, a good lens at 70-200mm, and a mushy mess at 200-300mm (at least, my non-L was very soft at 300mm).

Also don't forget that by upgrading from 40D to FF, for the reach of the 300mm on APS-C that you're used to, you'll need a 480mm lens on your FF to frame the same. Set your current lens at 125mm on your 40D, and see the angle of view that you get. Can you live with that as your longest? Because that's all you'll get with a 200mm on FF, if you want longer then you'll have to get one of the 4 i mentioned earlier, and/or get some extenders.

So, do you need 300mm or longer? And do you need it to be sharp? Then 70-300L, 100-400L, or even consider the 300mm f/4L or 400mm f/5.6L.Personally I chose the 70-300L when i got sick of my 70-300 non-L, not least because it was the newest, fastest AF, best sealed, best IS and most compact of the 4 (and cheapest at the time too).

Random Orbits:
If you're considering the 70-300L, then it might be worth considering a 70-200 f/2.8 L II. It takes extenders well and will get you to 400mm @ f/5.6, which is similar in reach and IQ to the 100-400, with slower AF.

With the 70-300L as good as it is, I don't think it's worth looking at the 70-200 f/4 variants for most people.

Menace:
Why not keep the 40d and get a 70-200 (which ever you can afford) - so you still have the extra reach of the crop camera body. Get the 5d III kit, keep 17-40 and 100 Macro and sell the rest.

You can always add a longer lens later on if you really really need it.